4.
Snubber design (RC snubber)
RC snubber Secondary side diode
T1 D202 L201
EER3016 MBRF10100
12V, 2.5A
1 +10 C201 C202
1000uF 1000uF
C104 25V 25V
R103
10nF 2 -8
56k
R102 1kV
2W D101
C103
30k
100uF UF 4007
3
400V
BD101 2 R105
2KBP06M3N257
40k IC1
FSDM07652R
6
1 3 Vstr 1
Drain
5 D201
NC L202
MBRF1045
Vcc 3
4 5V, 2A
4 Vfb 4 7
GND C105 D102 R104 C203 C204
C102 C106
2 ZD101 22uF TVR10G
5 1000uF 1000uF
220nF 47nF
22V 50V 10V 10V
275VAC 50V
6
5
C301
LF101 4.7nF
23mH
R201
1k
R101 R204
560k
5.6k
1W R202
R203 C205
1.2k
1.2k 47nF
IC301
H11A817A
F1 IC201
C101
RT1 FUSE KA431
220nF R205
5D-9 250V
275VAC
5.6k
2A
4. Snubber design (RC snubber)
Why Diode voltage oscillates when turned off?
Oscillation between the leakage inductance and diode output capacitance
Need to be damped with additional resonant network
Cout
Llk
Llk Llk
3Cout
Cout Z Cout
Llk Llk Cout 1
R= = =
Cout Cout 2f r Cout
4. Snubber design (RC snubber)
(1) Measure the original resonance period (Tr) (2) Find a capacitor value that doubles the
of the diode voltage waveforms. resonance period when connected in
parallel with the diode.
Csn
D202
MBRF10100
10
C201
1000uF
25V
8
Tr=46ns With 680pF capacitor, the resonance period is
approximately doubled (46ns 96ns)
4. Snubber design (RC snubber)
680pF 30
(3) Calculate the snubber resistor with the following equation.
D202
MBRF10100
10
C201
1000uF
25V
8
4. Snubber design (RCD snubber)
RCD snubber Primary side
4. Snubber design (RCD snubber)
, fs=66kHz)
Experimental results (Csn=2.2nF, Rsn=56k
Vsn 2 122 2 Vsn 2 1432
= = 0.266W = = 0.365W
Rsn 56k Rsn 56k
1 Vsn 1 122 1 Vsn 1 143
Llk I pk 2 f s = (3u ) 1.12 66k = 0.256W Llk I pk 2 f s = (3u ) 1.412 66k = 0.360W
2 Vsn VRO 2 122 65 2 Vsn VRO 2 143 65
4. Snubber design (RCD snubber)
Design Procedure
Measure the leakage inductance : measure the primary side
inductance with all other windings shorted
LCR meter is not always correct (Normally 50% error) especially the
leakage inductance is small
Determine the snubber capacitor voltage (Vsn) considering voltage
margin of BVdss
Calculate the snubber resistor using 2V (V V )
Rsn = sn sn 2RO
Llk f s I pk
Determine the snubber capacitor considering the snubber capacitor
voltage ripple (1~10nF)
If the measured drain voltage is different from the designed value,
leakage inductance should be calibrated using
2Vsn (Vsn VRO )
Llk =
Rsn f s I pk 2
4. Snubber design (RCD snubber)
Design Example (VRO=65V, Vin=265Vac(VDC=370V), Ipk=1.5A)
Measure the leakage inductance with LCR meter: 5uH @ 70kHz
Determine the snubber capacitor voltage considering voltage margin
of BVdss : Vsn=182V (Vsn+VDC=182+370=552V :85% of 650V)
Calculate the snubber resistor
2Vsn (Vsn VRO ) 2 182 (182 65)
Rsn = 2
= 2
= 57 k
Llk f s I pk 5u 66k 1.5
4. Snubber design (RCD snubber)
Determine the snubber capacitor considering the snubber capacitor
voltage ripple : Csn=2.2nF
Measured the peak drain voltage : 520V (Vsn=146V)
, Csn=2.2nF
Rsn=56k
Vsn=520-370=150V
# Vsn is different from the
designed value
Recalculate the leakage inductance with measured Vsn
2Vsn (Vsn VRO ) 2 150 (150 65)
Llk = 2
= 2
= 3uH
Rsn f s I pk 56k 66k 1.5
4. Snubber design (RCD snubber)
Recalculate snubber resistor
2Vsn (Vsn VRO ) 2 182 (182 65)
Rsn = 2
= 2
= 95k
Llk f s I pk 3u 66k 1.5
, Csn=2.2nF
Rsn=96k
Vsn=543-370=173V
The drain voltage is lower than the designed value by 9V due to the
stray resistance (Measured:543V, Designed:552)
